This invention relates generally to a system for processing electrical signals and in particular to apparatus for aligning brushes of a dynamoelectric machine to minimize arcing of the brushes.
In dynamoelectric machines such as DC generators, brushes are used to conduct current to and from collector slip rings or commutators attached to a rotor. Typically the brushes are mounted in a stationary brushholder assembly or rigging such as is shown for a large generator in U.S. Pat. No. 3,864,803 to Ohmstedt et al of common assignee to the present invention. This assembly supports the brushes and allows them to ride stably on the slip ring surface of the rotor. Although the brushholder rigging is held in fixed position during operation of the dynamoelectric machine, a small amount of rotational adjustment is often provided to permit varying of the commutation angle of the brushes for optimum performance of the dynamoelectric machine. In particular, the rigging should be set at a position such that arcing of the brushes during operation of the machine is avoided since, as is well known, arcing between the brushes and rotor slip ring results in electrical losses and can lead to damage to rotor slip rings, brushes, and brushholder riggings, forcing outages and repairs.
For most machines the total art length through which the brushholder rigging may be rotated without resulting in brush arcing is quite small, e.g., less than an inch even for large generators of power ratings up to 3000 kilowatts. The optimum rigging position within this narrow arc-free region, which is known as the "black band" because of the absence of visible brush arcing when the machine is operated in this region, is normally determined for a generator prior to shipment by means of tests known as "buck" and "boost" tests. During these tests, which may be conducted at part generator load or at full load, current is applied to the generator field either in excess of that normally furnished by the excitation system ("boost" current) or less than that normally furnished ("buck" current), and output signals from the brushes are measured by a meter. In general the brush output signals rise with increasing magnitude of buck or boost current applied, defining, when output signal is plotted against applied current, a curve approximately parabolic in shape whose minimum value of output signal may occur at a valve of applied current on either side of the electrical neutral, the point at which only normal excitation current (no buck or boost current) is being applied to the generator field. The two end points of the parabolic-shaped curve are defined by levels of buck and boost current and output signal at which the brushes begin arcing as determined optically or by means of a brush arc monitor such as is described in U.S. application Ser. No. 901,579, filed May 1, 1978, now U.S. Pat. No. 4,163,227, by Sawada et al and of common assignee to the present invention. For each brushholder rigging setting a different parabolic-shaped curve may be generated, and the optimum rigging setting is determined as that for which the minimum output signal occurs at, or displaced a prescribed increment of current from, electrical neutral, and also as that which has substantially the maximum arc-free range of buck and boost currents. These criteria assure minimum brush arcing during normal operation of the dynamoelectric machine and thus long brush and slip ring lives.
Prior to the present invention, the above-described output signals obtained during brush alignment tests were fed to a current-measuring device known as an arc index meter, and arcing at the end points of the output curve was detected visually using mirrors placed near the brushes. Although used extensively in alignment of generator brushes, this arc index meter is deficient in certain respects. First, the onset of brush arcing, which must be determined to establish the arc-free range of operation, is not detectable using the arc index meter but must be located by other means. Also, during alignment of the brushes of certain generators, particularly those with high levels of inherent noise, the curves of brush output signal versus applied current resulting from use of the arc index meter include ill-defined regions and in particular lack the well-defined minimum value of output signal required for precise alignment of machine brushes. Further, the response time of this prior art device is long, making accurate readings difficult and time-consuming to obtain. Finally, this instrument lacks the range of sensitivity necessary to accurately align brushes for the wide variety of dynamoelectric machine sizes and power ratings currently being manufactured.
Accordingly, it is an object of the invention to provide improved apparatus for aligning brushes of a dynamoelectric machine for minimum arcing of the brushes.
It is another object of the invention to provide apparatus for quickly and accurately aligning brushes for dynamoelectric machines in a wide range of sizes and power ratings.
It is a further object of the invention to provide apparatus which, in addition to facilitating alignment of brushes of a dynamoelectric machine, also is operable to detect the onset of brush arcing.